It should be obvious. Because it's the easiest thing for them to do. If it wasn't they wouldn't do it. Figure there is nothing there to stop them in the most cases and you can't slow them down so they just keep going. There is nothing that acts as a light resistor. There are things that let some light pass and other frequencies through like a filter. things that will stop them all together anything opaque. Which pretty much just transforms light energy into micro amounts heat energy. So once a photon is created the energy it was created with is always and will always be there unless something stops it altogether. It doesn't lose any of it's potential energy as it travels because there is no opposing force that can act on it unless it can transform the light back into physical energy in the form of heat.